Ammonia (NH3) and hydrogen sulfide (H2S) are frequently found together in by-product mixtures from petroleum refining processes such as hydrotreating, cracking, and coking, as well as in ore reduction, metal refining, paper production, and gasification of coal, coke, residual oils, and the like.
When NH3 and H2S are found in aqueous liquid streams, a sour water stripper is often employed to remove the NH3 and H2S from the water, producing a gaseous stream of H2S and NH3, which must be treated. In the past, NH3 and H2S by-products were incinerated or burned as fuel. However, combustion results in the formation of nitrogen and sulfur oxides which are corrosive and contribute to atmospheric pollution.
One method of processing gas streams containing a mixture of H2S and NH3 is to convert the H2S and NH3 into ammonium thiosulfate (ATS), (NH4)2S2O3. ATS is a commercially marketable product which can be sold to offset the costs of treating NH3 and H2S.
In a previous process for converting NH3 and H2S to ATS, a gaseous stream comprising NH3 and H2S was fed to a first contact zone in which it was contacted with a solution of ATS and ammonium sulfite. A second part of the solution is fed in parallel to a second contact zone wherein it absorbs SO2 from a second gas stream. In this formation, the contact zones were operated independently so that the first absorbed ammonia while selectively rejecting some H2S. The other contact zone absorbed SO2. However, this process requires high liquid rates, which reduce the energy and cost efficiency of the process.
ATS production processes can also produce elemental sulfur as a by-product. Elemental sulfur precipitates from an aqueous solution and must be removed from the system and disposed of, increasing the costs of treating the gas stream. The sulfur produced as elemental sulfur also reduces the amount of sulfur available for ATS production.
Consequently, there is a need for an efficient process of producing ammonium thiosulfate utilizing waste ammonia without the production of elemental sulfur.